Print tone measuring method

ABSTRACT

A print tone measuring method includes a reference paper image pickup step for reading an image of reference paper to obtain first image data, a designated control point setting step for setting designated control points, a first reference point setting step for setting a pair of first reference points for specifying image positions, a print reading step for reading an image of an actual print to obtain second image data, a second reference point setting step for setting a pair of second reference points corresponding to the first reference points, a calculating step for calculating positions of corresponding control points corresponding to the designated control points from the first reference points and the second reference points, an extracting step for extracting image data adjacent each of the designated control points and image data adjacent each of the corresponding control points, and a correcting step for correcting the position of each corresponding control point in the second image data by comparing the image data adjacent each designated control point and the image data adjacent each corresponding control point.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a print tone measuring method.

2. Description of the Related Art

In order to perform proper printing with a printing machine, it isnecessary to control ink feeding rates properly. For controlling the inkfeeding rates, it has been conventional practice to measure densities ofcontrol strips with a densitometer and determine from density datawhether the ink feeding rates are proper or not. However, the densitydata from the control strips alone is not necessarily sufficient forattaining a proper color tone and the like for a picture area.

For this reason, a print tone measuring apparatus is used which providescontrol data for controlling the ink feeding rates of a printingmachine. The control data is produced by comparing an image on referencepaper and an image on an actual print.

The reference paper is also called proof paper, and serves as areference indicating a color tone of finished prints to obtain properprints. Printing paper actually printed is also called sampling paperwhich is extracted by the operator from a discharge station of aprinting machine at certain intervals during a printing operation. Theprinting is considered proper when the color tone of the sampling papersubstantially coincides with the color tone of the reference paper.

Where such a print tone measuring apparatus is used, it is difficult toattain an agreement in color tone and the like in all areas between theimage on the reference paper and the image on the actual print. For thisreason, use is made of control points such as representative pointshaving colors that characterize a picture pattern on the print. Thesecontrol points are key points used in order to control the ink feedingrates, and are set to the image by the operator.

The control points must be set to accurately corresponding positions onthe image of the reference paper and the image of the actual print.Japanese Unexamined Patent Publication No. 10-305562 (1998) discloses aprint evaluating apparatus, in which a pair of reference points arespecified on each of an image of reference paper and an image of anactual print for indicating image positions, and positions of controlpoints on the image of the actual print are calculated from thesereference points and the control points on the image of the referencepaper.

With the above method, however, a partial distortion of the image of theprint makes it impossible to attain an accurate correspondingrelationship between the control points on the image of the referencepaper and those on the image of the print. That is, a curvatureaberration of an image pickup optical system for reading images from thereference paper and print, or an error due to a distortion of printingpaper, results in a displacement between the control points on the imageof the reference paper and those on the image of the print. Such adisplacement hampers an accurate measurement of the print tone.

An image area usually is formed of halftone dots. Particularly wherelarge halftone dots are used, a disagreement may occur between colorinformation on a single pixel and color information on an image formedof a plurality of halftone dots. Noise of a CCD camera acting as animage pickup device, or dust or scratches on the print, may also hamperan accurate measurement based on color information on a single pixel.

SUMMARY OF THE INVENTION

The object of this invention, therefore, is to provide a print tonemeasuring method capable of attaining an accurate correspondingrelationship between control points on an image of reference paper andthose on an image of an actual print, thereby enabling an accuratemeasurement of the print.

The above object is fulfilled, according to this invention, by a printtone measuring method for obtaining data for controlling ink feedingrates of a printing machine by comparing data of control points in imagedata obtained by reading an image of reference paper and data of controlpoints in image data obtained by reading an image of an actual print,the method comprising:

a reference paper reading step for reading the image of the referencepaper to obtain first image data;

a designated control point setting step for setting designated controlpoints to the first image data for use in measuring the print, thedesignated control points being set to respective regions on thereference paper corresponding to ink keys of the printing machine;

a first reference point setting step for setting a pair of firstreference points to the first image data for specifying image positions;

a print reading step for reading the image of the actual print to obtainsecond image data;

a second reference point setting step for setting a pair of secondreference points to the second image data to correspond to the firstreference points;

a first calculating step for calculating, from the first referencepoints and the second reference points, positions of correspondingcontrol points in the second image data corresponding to the designatedcontrol points in the first image data, for the respective regionscorresponding to the ink keys of the printing machine;

an extracting step for extracting image data adjacent each of thedesignated control points and image data adjacent each of thecorresponding control points;

a correcting step for correcting the position of each of thecorresponding control points in the second image data by comparing theimage data adjacent each of the designated control points and the imagedata adjacent each of the corresponding control points;

a first selecting step for comparing image data including each of thedesignated control points, of each of the regions corresponding to theink keys of the printing machine, among the image data adjacent thedesignated control points in the first image data, with a plurality oftemplates having different pixel arrangements, selecting a templateclosest to the image of the region including each of the designatedcontrol points, and selecting pixels corresponding in position to pixelsarranged in the selected template, as relevant pixels in the first imagedata;

a second selecting step for selecting, from the corresponding controlpoints corrected, positions of relevant pixels in the second image datacorresponding to the relevant points in the first image data, for therespective regions corresponding to the ink keys of the printingmachine; and

a second calculating step for calculating data for controlling the inkfeeding rates of the printing machine by comparing color values of therelevant pixels in the first image data and color values of the relevantpixels in the second image data for the respective regions correspondingto the ink keys of the printing machine.

This print tone measuring method corrects the positions of thecorresponding control points by comparing the image data adjacent thedesignated control points and the image data adjacent the correspondingcontrol points. Thus, an accurate agreement is achieved between thecontrol points in the image of the reference paper and those in theimage of the actual print, thereby enabling an accurate measurement ofthe print tone.

In another aspect of the invention, a print tone measuring method isprovided for measuring tones of an actual print by comparing data ofcontrol points in reference image data and data of control points inimage data obtained by reading an image of the actual print, the methodcomprising:

a first storing step for storing the reference image data as first imagedata;

a second storing step for storing the image data obtained by reading theimage of the actual print as second image data;

a reference point setting step for setting first reference points to thefirst image data, and second reference points to the second image datato correspond to the first reference points;

a designated control point setting step for setting designated controlpoints to one of the first image data and the second image data for usein measuring the print;

a calculating step for calculating, from the first reference points andthe second reference points, positions of corresponding control pointsin the other of the first image data and the second image datacorresponding to the designated control points in the one of the firstimage data and the second image data;

an extracting step for extracting image data adjacent the designatedcontrol points and image data adjacent the corresponding control points;and

a correcting step for correcting the positions of the correspondingcontrol points by comparing the image data adjacent the designatedcontrol points and the image data adjacent the corresponding controlpoints.

In a further aspect of the invention, a print tone measuring method isprovided for obtaining data for controlling ink feeding rates of aprinting machine by comparing first image data serving as a referenceand second image data obtained by reading an image of a print actuallymade by the printing machine, the method comprising:

a control point setting step for setting positions of control points tothe first image data;

an extracting step for extracting image data adjacent the control pointsfrom the first image data;

a selecting step for comparing image data including the control pointsamong the image data adjacent the control points in the first imagedata, with a plurality of templates having different pixel arrangements,selecting a template closest to the image of the region including thecontrol points, and selecting pixels corresponding in position to pixelsarranged in the selected template, as relevant pixels in the first imagedata;

a print reading step for reading the image of the actual print to obtainsecond image data; and

a calculating step for calculating data for controlling the ink feedingrates of the printing machine by comparing color values of the relevantpixels in the first image data and color values of relevant pixels inthe second image data corresponding to the relevant pixels in the firstimage data.

With this print tone measuring method, color may be measured accuratelyeven where large halftone dots are used, noise occurs with a CCD camera,or dust or scratches is/are present on the print.

Other features and advantages of the invention will be apparent from thefollowing detailed description of the embodiments of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

For the purpose of illustrating the invention, there are shown in thedrawings several forms which are presently preferred, it beingunderstood, however, that the invention is not limited to the precisearrangement and instrumentalities shown.

FIG. 1 is a perspective view of a print tone measuring apparatusaccording to this invention;

FIG. 2 is a side view of the print tone measuring apparatus;

FIG. 3 is a block diagram showing a principal structure of a controlunit;

FIG. 4 is a flow chart of a print tone measuring operation;

FIG. 5 is an explanatory view showing an image of reference paperdisplayed on a control panel;

FIG. 6A is an explanatory view showing image data A adjacent adesignated control point and image data B adjacent a correspondingcontrol point;

FIG. 6B is an explanatory view showing image data A adjacent thedesignated control point and image data B adjacent the correspondingcontrol point;

FIG. 6C is an explanatory view showing image data A adjacent thedesignated control point and image data B adjacent the correspondingcontrol point;

FIG. 7 is a flow chart of a print tone measuring operation in a secondembodiment;

FIG. 8 is an explanatory view showing an image adjacent a designatedcontrol point P displayed on the control panel;

FIG. 9A is a schematic view of a template;

FIG. 9B is a schematic view of a template;

FIG. 9C is a schematic view of a template;

FIG. 9D is a schematic view of a template;

FIG. 9E is a schematic view of a template;

FIG. 9F is a schematic view of a template;

FIG. 9G is a schematic view of a template; and

FIG. 9H is a schematic view of a template.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

An embodiment of this invention will be described hereinafter withreference to the drawings. FIG. 1 is a perspective view of a print tonemeasuring apparatus in a first embodiment of the invention. FIG. 2 is aside view of the apparatus. It is to be noted that light sources 13 anda control panel 15 are omitted from FIG. 2.

This print tone measuring apparatus is used to create control data forcontrolling ink feeding rates of a printing machine by measuring imagetones of reference paper and actual prints. The print tone measuringapparatus includes a table 12 disposed above a frame 11, a pair of lightsources 13 arranged at right and left sides of the table 12, an imagepickup unit 14 disposed above the table 12, a control panel 15 disposedabove one of the light sources 13, an upper light-shielding plate 17 anda rear light-shielding plate 18 supported by a pair of posts 16, anauxiliary light source 19 attached to the rear light-shielding plate 18,and a control unit 20 mounted inside the frame 11 for controlling theentire apparatus.

The table 12 is shaped planar for receiving a print thereon. The table12 has a surface in the form of a suction plate for holding the print bystatic electricity or vacuum suction. The surface of the table 12 isinclined about 10 degrees for facility of operation by the operator. Theprint held by suction on the inclined surface of the table 12 isilluminated by the pair of light sources 13 arranged at the oppositesides.

The image pickup unit 14 disposed above the table 12 has a digitalcamera for separating, with a dichroic mirror, light emitted from thelight sources 13 and reflected from the surface of the print into thethree primary color components of RGB, and receiving the individualcomponents with separate CCD arrays. With this image pickup unit 14, RGBdata can be obtained from the print.

The control panel 15 is the touch panel type in the form of an LCDmonitor having a pressure sensitive input function (also called a touchsensitive screen). This control panel 15 acts as both a display deviceand an input device, and is connected to the control unit 20 describedhereinafter.

FIG. 3 is a block diagram showing a principal structure of the controlunit 20.

This control unit 20 includes a ROM 21 for storing operating programsnecessary for controlling the apparatus, a RAM 22 for temporarilystoring data and the like during a control operation, a CPU 23 forperforming logic operations, and a first and a second image memories 24and 25. The control unit 20 is connected through an interface 26 to thecontrol panel 15, light sources 13 and image pickup unit 14 noted above.The control unit 20 is connected also to an image data source 27 storingimage data to be printed, such as a hard disk or an image processingdevice.

Referring again to FIGS. 1 and 2, the upper light-shielding plate 17supported by the pair of posts 16 has a curved configuration extendingin the fore and aft direction of the print tone measuring apparatus. Thelight-shielding plate 17 is installed in order to intercept light, suchas light from indoor light sources, that would constitute a regularreflection from the table 12. On the other hand, the rearlight-shielding plate 18 supported between the pair of posts 16 servesto intercept light coming from behind the print tone measuringapparatus.

The auxiliary light source 19 attached to the rear light-shielding plate18 serves to compensate for a lack of light on the table 12 caused bythe upper light-shielding plate 17 and rear light-shielding plate 18.The auxiliary light source 19 is in the form of a fluorescent light orthe like, which is turned off when reading an image of a print with theimage pickup unit 14.

A print tone measuring operation using the apparatus having the aboveconstruction will be described next. This operation is carried out toobtain data for controlling ink feeding rates of a printing machine.FIG. 4 is a flow chart of the print tone measuring operation.

To obtain data for controlling the ink feeding rates of the printingmachine, the image of the reference paper is first read by using theprint tone measuring apparatus (step S11). Specifically, the referencepaper is placed on the table 12 and held thereon by suction. Thereference paper is illuminated by the light sources 13, and the image ofthe reference paper is read by the image pickup unit 14. Data of theimage of the reference paper is stored as first image data in the firstimage memory 24 of the control unit 20 (step S12).

The image data of the reference paper stored in the first image memory24 is displayed on the control panel 15. FIG. 5 is an explanatory viewshowing the image of the reference paper displayed on the control panel15. In this embodiment, ink keys of the printing machine are dividedinto seven regions. Thus, seven regions corresponding to the ink keysare set to the image of the reference paper.

Next, control points are designated, which are key points forcontrolling the ink feeding rates (step S13). That is, based on theimage of the reference paper displayed on the control panel 15, theoperator designates control points P1-P7 (which may be collectivelycalled “designated control points P”) in the respective regionscorresponding to the ink keys as shown in FIG. 5. These control pointsconsist of representative points having colors that characterize apicture pattern on a print. The control points may be set automatically.In this specification, the control points set by the operator asdescribed above are called designated control points. Information on thedesignated control points P1-P7 is stored in the first image memory 24or RAM 22 of the control unit 20.

Next, a pair of first reference points T1 and T2 are designated forspecifying image positions in the first image data (step S14). As shownin FIG. 5, the first reference points T1 and T2, preferably, are set topoints on a diagonal line in the image. Information on these firstreference points T1 and T2 is stored in the first image memory 24 or RAM22 of the control unit 20. Step S13 and step S14 described above may beexecuted in reverse order.

Next, an image of an actual print is read (step S15). Specifically, aprint extracted by the operator from a discharge station of the printingmachine during a printing operation is placed on the table 12, and heldthereon by suction. This print is illuminated by the light sources 13,and the image of the print is read by the image pickup unit 14. Data ofthe image of the print is stored as second image data in the secondimage memory 25 of the control unit 20 (step S16).

The image data of the print stored in the second image memory 24 isdisplayed on the control panel 15 as is the image data of the referencepaper. Then, a pair of second reference points T3 and T4 are designatedto correspond to the pair of first reference points T1 and T2 designatedin the first image data (step S17). Information on these secondreference points T3 and T4 is stored in the second image memory 25 orRAM 22 of the control unit 20.

Then, control points on the image of the actual print corresponding tothe positions of the above designated control points P1-P7 arecalculated (step S18). Specifically, positions of the correspondingcontrol points are determined by geometrical calculation from positiondata of the first reference points T1 and T2, second reference points T3and T4 and designated control points P1-P7 determined previously. Sevensuch corresponding control points are set to the respective regionscorresponding to the ink keys of the printing machine, as are thedesignated control points P1-P7 shown in FIG. 5. Information on thesecorresponding control points is stored in the second image memory 25 orRAM 22 of the control unit 20.

This corresponding control point calculating step should provide therespective designated control points and corresponding control pointshaving coordinates positions in agreement. However, a partial distortionof the image of the print could inhibit an accurate agreement betweenthe coordinates of the designated control points on the reference paperand the coordinates of the corresponding control points on the print. Inthe print tone measuring method according to this invention, therefore,the following steps are executed to achieve an accurate agreementbetween the coordinates of the designated control points on thereference paper and the coordinates of the corresponding control pointson the print.

To achieve an agreement between the coordinates of the designatedcontrol points and the coordinates of the corresponding control points,image data adjacent each designated control point and image dataadjacent the corresponding control point are extracted (step S19).

FIGS. 6A through 6C are explanatory views showing image data A adjacenta designated control point and image data B adjacent a correspondingcontrol point.

As shown in these figures, image data A adjacent the designated controlpoint includes 169 pixels of each of R, G and B in an arrangement of 13rows by 13 columns. On the other hand, image data B adjacent thecorresponding control point includes 81 pixels of each of R, G and B inan arrangement of 9 rows by 9 columns. The position of the correspondingcontrol point is corrected by comparing these image data A and B (stepS20). In this embodiment, image data A is larger in size than image dataB in order to simplify an operating algorithm by causing the two imagedata to overlap each other constantly when determining differencesbetween the pixels while shifting the image data pixel by pixel. Thus,there is no limitation in the sizes of pixel data A and B. The step ofcorrecting the position of the corresponding control point is performedas follows.

As shown in FIG. 6A, the image data B adjacent the corresponding controlpoint is arranged relative to the image data A adjacent the designatedcontrol point with the respective upper and left ends placed inregister. In this state, differences (sums of R, G and B) between allthe pixels in the image data B and corresponding pixels in the imagedata A are added up and the resulting value is stored. This cumulativeaddition is carried out, with the differences not taking negative valuesbut taking absolute values.

Next, as shown in FIG. 6B, the image data B is shifted by one pixel tothe right. Then, differences between all the pixels in the image data Band corresponding pixels in the image data A are added up and theresulting value is stored. This operation is repeated 25 times by movingthe image data B rightward or downward by one pixel at a time.Consequently, as shown in FIG. 6C, the image data A and image data Bassume an arrangement with the respective lower and right ends inregister.

Then, a minimum value is determined among the 25 cumulative sumsacquired by the above matching operation. The position of image data Badjacent the corresponding control point providing the minimum value isa position of an accurate corresponding control point. Thus, a necessarycorrection is made to the information on the corresponding control pointpreviously stored in the second image memory 25 or RAM 22 of the controlunit 20. The coordinates of the designated control point and thecoordinates of the corresponding control point are thereby brought intoaccurate agreement even when a partial distortion occurs with the imageof the print.

In the above matching operation, the image data A adjacent eachdesignated control point is extracted in 13 rows by 13 columns, and theimage data B adjacent the corresponding control point is extracted in 9rows by 9 columns. The position of the corresponding control point maybe corrected even if the coordinates of the designated control point andthe coordinates of the corresponding control point are displacedrelative to each other by two pixels. Where the image data A adjacenteach designated control point is extracted in 15 rows by 15 columns, forexample, the position of the corresponding control point may becorrected even if the coordinates of the designated control point andthe coordinates of the corresponding control point are displacedrelative to each other by three pixels.

After achieving an accurate agreement between the coordinates of thedesignated control points and the coordinates of the correspondingcontrol points as described above, control data for controlling the inkfeeding rates of the printing machine is calculated for the respectiveregions corresponding to the ink keys of the printing machine by usingcolor data of the designated control points and color data of thecorresponding control points (step S21). This control data istransmitted on-line or off-line through the interface 26 shown in FIG. 3to the printing machine not shown. The above matching operation isdescribed by way of example only, and a different pattern matchingtechnique (calculation) may be used.

Then, whether a required number of prints have been made is determined(step S22). When the required number of prints has not been made, theoperation in step S15 to step S21 is repeated. At this time, theoperator extracts a print, for image reading, from the discharge stationof the printing machine upon completion of every 100 prints, forexample. When the required number of prints has been made, the operationis terminated.

In the embodiment described above, image data obtained by reading theimage of the reference paper is used as the image data for reference.Instead, the image data supplied from the image data source 27 forplatemaking purposes may be used as the reference image data. Thedescribed order of image reading from the reference paper and print maybe reversed.

In the embodiment described above, the designated control points are setto the image of the reference paper, to correct the coordinates of thecorresponding control points on the image of the actual print. Instead,designated control points may be set to the image of the actual print,to correct the coordinates of the corresponding control points on theimage of the reference paper.

A print tone measuring method in a different embodiment of the inventionwill be described next. FIG. 7 is a flow chart showing a print tonemeasuring method in a second embodiment.

In the foregoing embodiment, after achieving an accurate agreementbetween the coordinates of the designated control points and thecoordinates of the corresponding control points, control data forcontrolling the ink feeding rates of the printing machine is calculatedfor the respective regions corresponding to the ink keys of the printingmachine by using color data of the designated control points and colordata of the corresponding control points.

However, an image area usually is formed of halftone dots, andparticularly where large halftone dots are used, a disagreement mayoccur between color information on a single pixel and color informationon an image formed of a plurality of halftone dots. Noise of a CCDcamera acting as an image pickup device, or dust or scratches on theprint, may also hamper an accurate measurement based on colorinformation on a single pixel.

In the second embodiment, after correcting the positions of thecorresponding control points as in the first embodiment described above,image data of a region including a designated control point among theimage data adjacent the designated control point in the first image datais compared with a plurality of templates having different pixelarrangements. Pixels present in the same positions as the pixelsarranged in a template are selected as relevant pixels in the firstimage data. Thereafter, data for controlling the ink feeding rates ofthe printing machine is calculated for the respective regionscorresponding to the ink keys of the printing machine by comparing anaverage of colors of the relevant pixels in the first image data and anaverage of colors of relevant pixels in the second image data.

In the second embodiment, a relevant pixel selecting step (step S41) isexecuted after steps S31 to S40 which are the same as steps S11 to S20in the first embodiment. This relevant pixel selecting step is executedas follows.

First, image data adjacent each designated control point P is displayedon the control panel 15. FIG. 8 is an explanatory view showing an imageadjacent the designated control point P displayed on the control panel15. In FIG. 8, a linear pattern I is shown to extend in the transversedirection.

Next, relevant pixels relating to each designated control point P areselected. Specifically, image data including the designated controlpoint P among the image data adjacent the designated control point Pextracted previously is compared with a plurality of templates havingdifferent pixel arrangements, and a template closest to the image of aregion including the designated control point P is selected. Pixelspresent in the same positions as the pixels arranged in the selectedtemplate are selected as relevant pixels.

FIGS. 9A through 9H are schematic views showing eight templates havingdifferent pixel arrangements. These templates have regions arranged infive rows by five columns. In these figures, the regions having pixelsare hatched.

When selecting relevant pixels, as shown in FIG. 8, the center of atemplate L among the eight templates shown in FIGS. 9A-9H is placed tocoincide with the designated control point P. Then, an extent to whichthe positions of the pixels in this template L correspond to the patternI is determined by calculating distributions of the pixels. When, forexample, templates L having five pixels as shown in FIGS. 9E-9H areused, distribution Mr of the R color is derived from the followingequation:Mr=[(A−F)²+(B−F)²+(C−F)²+(D−F)²+(E−F) ²]/5

where A, B, C, D and E are pixel values of the five pixels, and F is anaverage of the pixel values A, B, C, D and E.

The same equation is used for deriving distributions Mg and Mb of the Gcolor and B color. When the templates L having seven pixels as shown inFIGS. 9A-9D are used, distributions Mr, Mg and Mb of RGB colors may bederived from a similar equation including seven terms corresponding tothe seven pixels.

Next, a final distribution M is derived from the following equation:M=(Mr+Mg+Mb)/3

While changing the eight templates L shown in FIGS. 9A-9H one afteranother, distributions M are obtained for all the eight templates Lshown in FIGS. 9A-9H. The template L providing a minimum distribution Mamong these distributions M is determined and selected as the closest tothe pattern adjacent the designated control point P extractedpreviously. In the case of the pattern I shown in FIG. 8, the template Lshown in FIG. 9F is selected. The pixels arranged in the selectedtemplate L are selected as relevant pixels relating to the designatedcontrol point P.

Next, image data adjacent each control point is displayed on the controlpanel 15. Then, relevant pixels relating to the corresponding controlpoints are selected from the image data adjacent the correspondingcontrol points. Specifically, the center of selected template L shown inFIG. 9F may be placed to coincide with a corresponding control point. Inthis state, pixels arranged in the same positions as the pixels in theselected template L shown in FIG. 9F are selected as relevant pixels.

When the relevant pixel selecting step (step S41) is completed by theabove operation, control data for controlling the ink feeding rates ofthe printing machine is calculated for the respective regionscorresponding to the ink keys of the printing machine by using colordata of the designated control points and color data of thecorresponding control points (step S42). At this time, an average of thecolors of the relevant pixels relating to the designated control pointsP is determined as color of the designated control points. An average ofthe relevant pixels relating to the corresponding control points isdetermined as color of the corresponding control points. That is, theaverage of the colors of the relevant pixels relating to the designatedcontrol points P is used as data of the color of the designated controlpoints, and the average of the relevant pixels relating to thecorresponding control points is used as data of the correspondingcontrol points. Thus, color may be measured accurately even where largehalftone dots are used, noise occurs with a CCD camera, or dust orscratches is/are present on the print.

Weighted averages may be used instead of an average of the colors of therelevant pixels relating to the designated control points P and anaverage of the relevant pixels relating to the corresponding controlpoints. In this case, values of the control points may be regarded ashigh and values of surrounding areas regarded as low, or values may beweighted according to distances from the control points.

The control data is transmitted on-line or off-line through theinterface 26 shown in FIG. 3 to the printing machine not shown.

Then, whether a required number of prints have been made is determined(step S43). When the required number of prints has not been made, theoperation in step S35 to step S42 is repeated. At this time, theoperator extracts a print, for image reading, from the discharge stationof the printing machine upon completion of every 100 prints, forexample. When the required number of prints has been made, the operationis terminated.

In this embodiment, image data of a region including a designatedcontrol point among the image data adjacent the designated control pointis compared with a plurality of templates having different pixelarrangements. A template closest to the image of the region includingthe designated control point is selected. Pixels present in the samepositions as the pixels arranged in the selected template are selectedas relevant pixels. However, of the image data adjacent each designatedcontrol point, pixels within a predetermined range of color differenceswith respect to the color of the designated control point may beselected as relevant pixels. In this case, step S41 shown in FIG. 7 maybe executed to select pixels in the positions corresponding to therelevant pixels selected in step S4, as relevant pixels relating to thecorresponding control point.

In the embodiment described above, image data obtained by reading theimage of the reference paper is used as the image data for reference.Instead, the image data supplied from the image data source 27 forplatemaking purposes may be used as the reference image data. Thedescribed order of image reading from the reference paper and print maybe reversed.

In the embodiment described above, the designated control points are setto the image of the reference paper, to correct the coordinates of thecorresponding control points in the image of the actual print. Instead,designated control points may be set to the image of the actual print,to correct the coordinates of the corresponding control points on theimage of the reference paper.

This invention may be embodied in other specific forms without departingfrom the spirit or essential attributes thereof and, accordingly,reference should be made to the appended claims, rather than to theforegoing specification, as indicating the scope of the invention.

This application claims priority benefit under 35 U.S.C. Section 119 ofJapanese Patent Applications No. 2003-166335 and No. 2003-166336 filedin the Japanese Patent Office on Jun. 11, 2003, the entire disclosure ofwhich is incorporated herein by reference.

1. A print tone measuring method for obtaining data for controlling inkfeeding rates of a printing machine by comparing data of control pointsin image data obtained by reading an image of reference paper and dataof control points in image data obtained by reading an image of anactual print, said method comprising: a reference paper reading step forreading the image of the reference paper to obtain first image data; adesignated control point setting step for setting designated controlpoints to said first image data for use in measuring the print, saiddesignated control points being set to respective regions on thereference paper corresponding to ink keys of the printing machine; afirst reference point setting step for setting a pair of first referencepoints to said first image data for specifying image positions; a printreading step for reading the image of the actual print to obtain secondimage data; a second reference point setting step for setting a pair ofsecond reference points to said second image data to correspond to saidfirst reference points; a first calculating step for calculating, fromsaid first reference points and said second reference points, positionsof corresponding control points in said second image data correspondingto said designated control points in said first image data, for therespective regions corresponding to the ink keys of the printingmachine; an extracting step for extracting image data adjacent each ofsaid designated control points and image data adjacent each of saidcorresponding control points; a correcting step for correcting theposition of each of said corresponding control points in said secondimage data by comparing the image data adjacent each of said designatedcontrol points and the image data adjacent each of said correspondingcontrol points; a first selecting step for comparing image dataincluding each of said designated control points, of each of the regionscorresponding to the ink keys of the printing machine, among the imagedata adjacent the designated control points in said first image data,with a plurality of templates having different pixel arrangements,selecting a template closest to the image of the region including eachof said designated control points, and selecting pixels corresponding inposition to pixels arranged in the selected template, as relevant pixelsin said first image data; a second selecting step for selecting, fromthe corresponding control points corrected, positions of relevant pixelsin said second image data corresponding to the relevant points in saidfirst image data, for the respective regions corresponding to the inkkeys of the printing machine; and a second calculating step forcalculating data for controlling the ink feeding rates of the printingmachine by comparing color values of the relevant pixels in said firstimage data and color values of the relevant pixels in said second imagedata for the respective regions corresponding to the ink keys of theprinting machine.
 2. A print tone measuring method as defined in claim1, wherein said second calculating step is executed to determine anaverage of the color values of the relevant pixels selected in saidsecond selecting step as a color of the control points.
 3. A print tonemeasuring method as defined in claim 2, wherein said first selectingstep is executed to calculate a distribution of pixels in the image ofthe region including each of said designated control points,corresponding to the pixels arranged in said templates, and to select atemplate closest to the image of the region including each of saiddesignated control points.
 4. A print tone measuring method as definedin claim 2, wherein said first selecting step is executed to select, asrelevant pixels, those pixels in the image data adjacent each of saidcontrol points, which are within a predetermined range of colordifferences with respect to a color of the control point.
 5. A printtone measuring method for obtaining data for controlling ink feedingrates of a printing machine by comparing first image data serving as areference and second image data obtained by reading an image of a printactually made by the printing machine, said method comprising: a controlpoint setting step for setting positions of control points to said firstimage data; an extracting step for extracting image data adjacent saidcontrol points from said first image data; a selecting step forcomparing image data including said control points among the image dataadjacent the control points in said first image data, with a pluralityof templates having different pixel arrangements, selecting a templateclosest to the image of the region including said control points, andselecting pixels corresponding in position to pixels arranged in theselected template, as relevant pixels in said first image data; a printreading step for reading the image of the actual print to obtain secondimage data; and a calculating step for calculating data for controllingthe ink feeding rates of the printing machine by comparing color valuesof the relevant pixels in said first image data and color values ofrelevant pixels in said second image data corresponding to the relevantpixels in said first image data; wherein said calculating step isexecuted to determine an average of the color values of the relevantpixels selected in said second selecting step as a color of the controlpoints; and said selecting step is executed to calculate a distributionof pixels in the image of the region including each of said designatedcontrol points, corresponding to the pixels arranged in said templates,and to select a template closest to the image of the region includingeach of said designated control points.
 6. A print tone measuring methodas defined in claim 5, wherein said selecting step is executed toselect, as relevant pixels, those pixels in the image data adjacent eachof said control points, which are within a predetermined range of colordifferences with respect to a color of the control point.